The present invention relates to a process for producing poly(butylene terephthalate) by the direct esterification of terephthalic acid and 1,4-butanediol.
It has long been known that 1,4-butanediol can be used as the diol component for the production of polyesters. The resulting products have been widely used because these polyesters have superior properties such as moldability, good surface characteristics, and the dimensional accuracy of molded products. Among such polyesters, the most useful one is poly(butylene terephthalate) which is effectively used as an engineering plastic for electrical components, machines, cars, sporting goods, interior decorative goods and the like.
The patent literature is replete with descriptions of preparing poly(alkylene terephthalates) by reacting an alkylene glycol with dialkyl terephthalates and/or terephthalic acid. If dialkyl terephthalate is used, the first stage of the process is an ester interchange reaction wherein the reaction is considered complete when the monohydric alcohol is substantially completely removed. If terephthalic acid is used, the first stage of the process is a polyesterification reaction. In this case, the first stage is considered complete when the cloudy reaction medium becomes clear. Terephthalic acid (TPA) is generally preferred for the production of poly(ethylene terephthalate) since terephthalic acid is less expensive on a molar basis than the dialkyl ester and because first stage polyesterification is faster than first stage ester interchange.
Numerous patents have disclosed the reaction of 1,4-butanediol with terephthalic acid to produce poly(butylene terephthalate). However, in this case, polyesterification is subject to a major disadvantage, viz., that terephthalic acid acts as a catalyst for the conversion of 1,4-butanediol to tetrahydrofuran (THF). This reduces yield and increases the production cost of the resin.
It has been proposed in U.S. Pat. No. 3,936,421 to minimize tetrahydrofuran production by esterification of terephthalic acid with 1,4-butanediol in the presence of a mixture of an organo-tin compound and an organo-titanium compound as catalyst. Example 1 shows that, by using 0.1 wt. % of tetrabutyl titanate as catalyst, the distilled water (23.0 g) contains 8.5 g of tetrahydrofuran (8.5/23=36.96% THF). Example 2 indicates that distilled water (1960 g) contains 670 g of tetrahydrofuran (e.g., 670/1960=34.18% THF).
U.S. Pat. No. 4,014,858 also discloses an attempt to minimize tetrahydrofuran production by using a tetravalent tin catalyst having one organo-tin linkage. In Examples X and XII listed in Table 1, Columns 5 and 6, 0.23 and 0.20 mole, respectively, of THF were formed per mole TPA charged to the reactor.
U.S. Pat. No. 4,329,444 teaches controlling conditions to promote rapid removal of water as formed to minimize the contact time between 1,4-butanediol and terephthalic acid. Example 1 indicates that, by using 0.18 wt. % of tetrabutyl titanate as catalyst, 0.358 mole diol may be converted to tetrahydrofuran per mole of terephthalic acid charged to the reactor. This implies that the amount of tetrahydrofuran formed was 0.358 mole THF/mole TPA. In Example 2, the amount of tetrahydrofuran (THF) formed was 0.63 mole THF/mole TPA.
U.S. Pat. No. 4,346,213 and U.S. Pat. No. 4,439,597 teach various techniques for conducting the polycondensation reaction prior to completion of the esterification/oligomerization reaction to improve yields.
Because of the disadvantage associated with the direct esterification method, the principal process for manufacturing poly(butylene terephthalate) remains the transesterification of a dialkyl terephthalate, generally dimethyl terephthalate, with 1,4-butanediol since less tetrahydrofuran is encountered by this procedure. This is so despite the work described in the aforesaid patents.